1. Field of the Invention
This invention relates to optical cables that are supported along the route of electrical power transmission system by means of towers, masts or other upstanding supports that are also employed to support electrical power cables.
2. Discussion of the Prior Art
In systems of this kind it is the general practice to earth the optical cable or cables at the towers, masts or other supports hereinafter simply referred to as towers. When the electrical power lines are under load, electrical currents may be capacitively induced on the optical cable due to the distributed capacitance between the cable and the power lines. The voltage induced on the optical cable will reach a maximum at mid-span between the towers, while the current flowing along the cable will be greatest in the region of the towers. Under dry conditions the induced currents will be relatively small due to the relatively high longitudinal resistance of the cable, e.g. in the region of 10.sup.12 ohm m.sup.-1 but under wet conditions when the surface resistance of the cable is much lower, e.g. in the region of 10 Mohm m.sup.-1, much higher currents will be induced. Joule heating of the cable surface by the induced currents can cause a short length of the cable surface to become dry, usually in the region of a tower where the current is highest. When this happens the major part of the induced voltage on the cable is dropped across the short dry band due to its high longitudinal resistance, and so called "dry-band arcing" may occur which can cause severe damage to the cable.
It is possible to overcome the problem of dry-band-arcing in an optical cable by providing the cable with a longitudinally extending electrically conductive path. However, an optical cable having such an electrically conductive path has the disadvantage that there are considerable safety issues to be taken into account if it is to be installed between towers of an overhead electric power transmissions line that is on load in view of the danger of it touching one of the transmission lines; moreover, it is not always possible or desirable to interrupt the electrical power transmitted by the overhead electric power transmission line for a time sufficient to enable such an optical cable to be installed.
It has been proposed, for example in European Patent Application No: 214,480, to employ a cable having a resistive element and a linear resistance of 10.sup.7 to 10.sup.12 ohm m.sup.-1. However, such systems have the disadvantages (among others) that the electrical properties of the resistive element may vary with time due to aging, pollution, cable strain and the like and as a result lose its efficacy.
It has also been proposed, for example in European Patent Application No. 403,285 to include a resistive flitting on the optical cable adjacent to the tower in order to reduce arcing on the cable and joule heating. However, such fittings do not eliminate the occurrence of stable dry-band arcing.